1. Field of the Invention
This invention relates generally to suspension systems and methods for isolating and reducing the transmission of vibratory motion between an object and a base. More particularly, the present invention is directed to a novel and improved horizontal-motion vibration isolation system that is nearly insensitive to the payload weight and can attain low natural frequencies. Additionally, this improved horizontal-motion vibration isolation system can be made fully adjustable and offers improved horizontal-motion performance while passively accommodating changes in payload weight and maintaining a low horizontal natural frequency. The vibration isolation system of the present invention also can be configured in a low profile without compromising horizontal-motion performance.
2. Description of Related Art
The problems caused by unwanted vibration on equipment, devices and processes that are extremely motion sensitive have been widely researched and numerous solutions to prevent or reduce the transmission of such vibratory motion have been proposed and developed. Many of the devices designed to reduce the transmission of unwanted vibration between an object and its surroundings, commonly called vibration isolators or suspension devices, have utilized various combinations of elements such as resilient pads made from a variety of materials, various types of mechanical springs, and pneumatic devices. There are, however, shortcomings and disadvantages associated with these particular isolation systems which prevent them from obtaining low system natural frequencies and from limiting resonant responses to low values while providing high isolation performance at higher frequencies.
The shortcomings and disadvantages of prior systems were addressed through my development of the novel vibration isolation systems and novel devices and methods shown and described in U.S. Pat. Nos. 5,310,157, 5,370,352, 5,178,357, 5,390,892, 5,549,270, 5,669,594 and 5,833,204, which are all hereby incorporated by reference. The particular vibration isolation systems described in these patents provide versatile vibration isolation by exhibiting low stiffness in an axial direction (generally in the direction of the payload weight) and any direction substantially transversed to the axial direction (generally a horizontal direction), and may provide tilt or rotation about three mutually perpendicular axes. These systems utilize a combination of uni-directional or bi-directional isolator subassemblies that can be connected together in series-fashion to provide omni-directional isolation. Each isolator is designed to isolate the axial or transverse component of any vibratory translation to effectively isolate vibrations along or about any directional axes.
These isolators rely on a particular principle of loading a particular elastic structure which forms the isolator or portion of it (the loading being applied by either the supported weight or by an external loading mechanism) to approach the elastic structure""s point of elastic instability. Some of my previous horizontal-motion isolation systems covered under U.S. Pat. Nos. 5,310,157, 5,370,352, 5,549,270, and 5,669,594 utilize a set of beam-columns, connected between upper and lower column plates, and loaded by the payload weight to approach the xe2x80x9ccritical buckling loadxe2x80x9d of the column. The critical buckling load is the load on the column at which the horizontal stiffness of the column is substantially reduced and approaches zero stiffness when the columns are loaded above their critical buckling load, the system becomes unstable. When the columns are loaded slightly below their critical buckling load, the horizontal stiffness and horizontal natural frequency are very low. While stiffness is greatly reduced, the columns still retain the ability to support the payload weight.
These critical buckling loads do not change significantly with changes in payload weight so that any substantial change in payload weight can cause changes in the horizontal stiffness and the horizontal natural frequencies of the isolation system. This results in a limited range of payload weight for which low horizontal natural frequencies can be achieved, e.g., 0.5 Hz or less. Also, the range of payload weight will decrease as the columns decrease in height These limitations were addressed in my previous horizontal-motion vibration isolation systems described, for example, in U.S. Pat. Nos. 5,178,357 and 5,390,892.
Accordingly, those concerned with the development and use of vibration isolation systems and apparatus recognize the need for improved systems and apparatus for achieving a horizontal-motion vibration isolation system having a horizontal natural frequency that can be made very low and that is nearly insensitive to changes in payload weight. The present invention satisfies these and other needs.
The present invention provides a horizontal-motion vibration isolation system that can attain low horizontal natural frequencies and is nearly insensitive to changes in payload weight. In the present invention, the critical buckling load of a column and the horizontal stiffness of the column are approximately proportional to the payload weight supported by the column. Through proper selection of the column parameters, the critical buckling load will remain slightly above the load on the column so that low horizontal natural frequencies result that remain nearly constant even as the payload weight changes. This feature allows the use of smaller column heights for achieving a wider range of payload weight for which the very low horizontal natural frequencies can be achieved.
The present invention includes a set of columns connected between upper and lower members, such as the upper and lower column plates or platforms. Each column of the present invention is a relatively rigid member connected to an upper plate and a lower plate by tilt mechanisms having a stiffness for tilt rotation about any horizontal axis. This tilt rotational stiffness is nearly proportional to the weight load carried by the column. Further, the column parameters can be selected so that the critical buckling load remains slightly above the load on the column, independent or nearly independent of the load on the column. Also, the length of the column can be adjusted, thereby changing the critical buckling load and horizontal natural frequency of the system.
There are various ways to achieve the column behavior and the resulting performance of the horizontal-motion vibration isolation system of the present invention. In one embodiment, a set of columns is connect between an upper plate and a lower plate. Each column comprises a threaded rod, with disks having central holes to accommodate the rod attached near its upper and lower ends with two lock nuts. A set of equally spaced tension members is connected at the outer edge of the upper disk and extends radially outward at some non-zero angle with the horizontal. These tension members are attached to an inner edge of a circular cutout in the upper plate. Another set of equally-spaced tension members is connected at the outer edge of the lower disk which also extends radially outward at some non-zero angle with the horizontal and are attached at the inner edge of a circular cutout in the lower plate. The tension members are very stiff in axial tension and very flexible in bending and torsion. The weight load from the payload and the upper plate loads the columns in compression and produces tension loads in the tension members. The upper set of tension members connected between the upper disk and the upper plate and the lower set of tension members connected between the lower disk and the lower plate also act as tilt rotational springs and produce resisting moments when the upper plate is translated in any horizontal direction relative to the lower plate. Since the bending and twisting stiffness of the tension elements is very small compared with the axial stiffness, this tilt rotational stiffness results primarily from the tension in the tension members. Further, since the tension in the tension members is proportional to the weight load on the column, the tilt rotational stiffness is therefore primarily a result of the weight load on the column.
As will be shown later with the aid of figures and a moment balance, a column behaving as a rigid member connected to upper and lower plates by tilt rotational springs having a stiffness proportional to the weight load on the column attains a critical buckling load that is proportional to the weight load on the column and is inversely proportional to the column""s length. Such a column also would have a horizontal stiffness that is proportional to the weight load on the column. A horizontal-motion vibration isolation system constructed from a set of such columns would have a natural frequency that is independent of the weight load and, by proper selection of the tilt rotational stiffness, the natural frequency can be made very low. It will also be shown with the aid of figures that this particular embodiment of the present invention approximates this behavior and that the proportionality constant between the tilt rotational stiffness and the weight load on the column is a function of the diameter of the disks and the angle the tension members make with the horizontal. Therefore, by proper selection of the disk diameter, along with the angle that the tension members make with the horizontal, and the column length, a column can be made so that the horizontal-motion vibration isolation system has a natural frequency that is nearly independent of the payload weight and can be made very low. Further, the natural frequency can be fully adjusted through the adjustment of the length of the column.
If these tension members elongate significantly as a result of the tension force, the angle that the tension members make with the horizontal, and hence the tilt rotational stiffness, will change with the payload weight. Therefore, the tension members should be very stiff in tension so their elongation under load is minimized, and they should also be very flexible in bending and twisting so that the tilt rotational stiffness is due primarily to the tension in the tension members and, hence, the weight load. Various elements can be used for the tension members, including, but not limited to, thin wires, strings, filaments, cables, thin and narrow sheet metal strips or other structural sheet material. The set of tension members can also be constructed as an integral member such as a wire mesh or an integral sheet metal stamping in the form of an inner and an outer ring connected by multiple thin radial elements.
Only three tension members with substantially equal angular spacing are actually needed to provide omnidirectional behavior, i.e., the same stiffness behavior in any horizontal direction. Multiples of three tension members, such as six or more, can also be used and when more than a few equally spaced members are used, the behavior should be approximately omnidirectional, independent of the number of tension members.
In another embodiment of the invention, a set of columns having relatively rigid members is connected between upper and lower column plates and the tilt rotational stiffness at the connections is provided by end fittings on the rigid members that are pressed into relatively deformable pads by the weight load on the column. Relative horizontal translation between the column plates causes tilting rotation of the column and deformation of the pads that produces a moment resisting the tilt rotation. The tilt rotational stiffness is equal to this moment divided by the tilt angle. The end fittings have a conical or other contoured shape so that as the weight load on the column increases, the contact area between the end fitting and the deformation of the pad increases. This increased contact area and deformation causes an increase in the tilt rotational stiffness. By proper selection of the shape of the pad, the shape of the end fitting contour and the material properties of the pad, the tilt rotational stiffness can be made approximately proportional to the weight load on the column. Various materials can be used for the pads. Examples are natural rubber, Neoprene and other rubber-like materials, metallic meshes and combinations of materials.
In one particular embodiment of the end fittings have threaded holes and screw on to a rod that has right-handed and left-handed threads at the ends. The spacing of the end fittings, and therefore the effective length of the column, can be changed by turning the rods. The pads are bonded to the column plates and mating protrusions on the end fittings and recesses in the pads anchor the end-fittings in the pads.
In another embodiment, the column assemblies are comprised of threaded rods with right-handed and left-handed threads at each end that screw into threaded fittings that connect to tapered coil springs connected to the upper and lower column plates. There are recesses in the column plates for locating the tapered coil springs. The weight loads on the columns compress the tapered springs. Increasing the weight loads on the tapered springs increase both the axial stiffness and the tilt rotational stiffness of the tapered springs by causing the larger more flexible coils to bottom out on adjacent coils or on the supporting surfaces of the column plates, thereby increasing both the axial and the tilt stiffness of the springs. By proper design of the tapered springs, the tilt rotational stiffness can be made approximately proportional to the weight load on the column. The tapered spring design variables include the material, wire diameter, small coil diameter, large coil diameter, number of coils, free length, axial spacing of the coils and diameters of the coils.
In another embodiment of the invention similar to the previous embodiment, three tapered coil springs spaced at 120 degrees are pressed between each end fitting and the column plate. There are recesses in the end fittings and in the column plates for housing the springs. The tilt rotational stiffness in this embodiment is a function of the axial and the tilt rotational stiffness of the tapered springs and their radial distance from the center of the threaded rod. By proper selection of the radial position and design of the tapered springs the tilt rotational stiffness can be made approximately proportional to the weight load on the column.